When you create a new account in eTapestry, the database will ask you to select a Role for the account:
 
Account Roles
 
Any given account in eTapestry can only have one defined role. An account can either be a User or a Constituent, for example, but it cannot be both. If you have a situation where an individual needs more than one role with the organization, eTapestry will require you two create two accounts for that person, one account for each role. For example if one of your employees uses the database but also donates to the organization, they would have both a Constituent account under their name (where the donations are tracked) and a User account (which they use to log in to the database). For directions on how to complete this process, please refer to How to add a role to an existing account.

Two accounts with separate roles cannot be merged together. A Constituent cannot be merged with a Tribute account, a Tribute cannot be merged with a User, so on and so forth.

If you are unable to find the second account when you're attempting to set up the merge, this account likely has a different role than the one you currently have open. If you have a Constituent account open and you click Merge Role to search for the second account, eTapestry will only show you Constituent accounts. If the second account for that individual is a Tribute account, you will not be able to find that account in your search.

You can verify this by searching for the second account and opening it, then looking in the upper right hand corner where the account number displays. eTapestry will display in this same section the role assigned for that account:
 
Account Role and Account Number
Please note this is planned behavior and not something that needs to be corrected; if an individual has more than one role with the organization, they should have separate accounts for each role.